1. Field of the Invention
The present invention relates to an excavating method for excavating an underground continuous ditch to construct an underground continuous wall to be used, for example, for water sealing or for foundation.
2. Description of the Prior Art
Heretofore, as an apparatus for excavating an underground continuous ditch, there has been known such an apparatus as shown in FIG. 6, in which a chain type cutter 4 is attached vertically to a traveling carrier car 1 through a main frame 2 and a leader 3, and with the cutter 4 positioned underground, the cutter is rotated while being allowed to move horizontally (traversing), to excavate a ditch G of a constant width continuously (see, for example, Japanese Patent Laid Open Nos. 280043/93,280044/93 and 173835/95).
The chain type cutter 4 comprises an endless chain 8 and a large number of excavating edges 9 formed on the outer periphery of the chain 8 to excavate the ditch G. The endless chain 8 is stretched between a driving wheel (sprocket) 6 disposed at the upper end of a cutter post 5 which is a vertically long, box-shaped frame and a floating wheel (pulley) 7 disposed at the lower end of the cutter post.
The cutter 4 is supported vertically movably with respect to the leader 3. It is moved vertically to adjust the depth by means of a lift cylinder (a hydraulic cylinder) 10 disposed between the leader 3 and the cutter 4 or by such a lift means as a winch (not shown).
The cutter 4 is also supported so as to be movable (traversing) in the horizontal transverse direction together with the leader 3 with respect to the main frame 2. The cutter 4 is pressed against a ground A (the thick arrow in FIG. 6 represents this pressing force) by means of upper and lower traversing cylinders (hydraulic cylinders) 11, 12 disposed between the main frame 2 and the leader 3.
The traversing force of the cutter 4 is created and imparted to the cutter mainly by the lower traversing cylinder 12. The upper traversing cylinder 11 functions to bear a pressing reaction force.
When the traversing cylinders 11 and 12 assume the state of maximum extension, both cylinders are contracted and at the same time the carrier car 1 advances in the excavating direction. By repeating this operation the ditch G is excavated continuously.
According to such a conventional method for excavating an underground continuous ditch, there arises the following problem because there is performed only the traversing excavation in which the ground A is excavated in reliance on the traversing force imparted to the cutter 4 by the traversing cylinders 11 and 12.
Generally, as the depth increases, the ground A tends to become harder and the frictional force between the cutter 4 and the wall of the excavated ditch G tends to increase.
On the other hand, with an increase of the depth, the traversing force (pressing force) capable of being exerted on the ground A by the cutter 4 becomes smaller, and when the depth exceeds a certain limit, the traversing force required for excavation is no longer imparted to the lower portion of the cutter.
Consequently, the movement of the lower portion of the cutter becomes slower than the upper portion thereof, so that the cutter as a whole is inclined or deflected forward. In this state, the lower portion of the ground A is left unexcavated, which eventually becomes a portion incapable of being excavated.
Thus, according to the conventional excavating method, an excavatable depth limit depth in excavation) is small.